The paired rectus abdominis muscles run side by side and adjacent to the midline, between the sternum and pubic bones, the lowest part of the abdomen. The rectus abdominis originates where the two pubic bones join (the pubic symphysis). The fibers run up to the end of the sternum (the xiphoid process) and the nearby surfaces of the 5th through 7th ribs. This muscle is unique in that there are tendons within the muscle. In most cases, a tendon is the link between a muscle and a bone, but the rectus abdominis has three tendons that break the muscle into distinct sections. When this muscle is well trained and the layer of fat under the skin is thin, the result is the highly sought after six-pack appearance associated with rock-hard abs.
The external oblique, as its name implies, is the outermost layer of the abdominal muscles that wrap around the lower abdomen. Its fibers run in a diagonal direction. It originates laterally on the outer surface of the lower 8 ribs, and the fibers run diagonally down toward the pelvis to insert on the iliac crest (that bony ridge on your side), the rectus sheath, and the linea alba.
The internal oblique lies just under the external oblique, and their fibers run perpendicular to each other. The internal oblique originates from the lumbodorsal fascia of the lower back and the adjacent iliac crest of the pelvis. Its fibers run diagonally up to the outer surfaces of the 9th through 12th ribs, the rectus sheath, and the linea alba.
The deepest abdominal muscle is the transversus abdominis. This muscle has a broad area of origin from the outer, lateral surface of the lower 6 ribs, the lumbodorsal fascia, and the iliac crest. Its fibers run horizontally to insert on the linea alba and rectus sheath. Don’t make the mistake of calling this the transversus abdominal oblique. The fibers are horizontal, not diagonal, so to add oblique would contradict the transversus in its name.
These three muscles connect to the linea alba by way of fairly long, flat tendons because the actual muscle tissue ends well lateral of the midline. The only muscles per se that are on either side of the navel are the paired rectus abdominis muscles.
Many people believe the abdominals collectively perform trunk flexion and trunk rotation. But when considering the direction of the muscle fibers, it is as hard for the rectus abdominis to aid in rotation as it is for the transversus abdominis to perform trunk flexion.
Since we know the direction of the fibers, the attachments, and the rule about muscles pulling the insertion toward the origin, the actions of the abdominal muscles are predictable, if complex. Also remember that these muscles can work with their partners on the opposite side or work alone. Let’s look first at the external oblique. When both external oblique muscles contract, they flex the trunk. When the muscle on the right side contracts, the trunk flexes laterally to the right. In addition, when the muscle on the right side contracts, the trunk can rotate toward the left.
The internal oblique is similar but has one main difference. Contract both sides to flex the trunk. Contract the muscle on the right side, and the trunk flexes laterally to the right. The difference is with rotation. Contract the muscle on the right side, and the trunk rotates to the right.
The transversus abdominis has different isolated actions. When activated, it increases intra-abdominal pressure and provides support for the abdominal organs.
The final abdominal muscle, the rectus abdominis, flexes the trunk and also helps perform lateral flexion and rotation.
Collectively, all four of these abdominal muscles work with each other and the long spinal muscles to provide support and stabilization for what many fitness professionals refer to as the lumbo–pelvic–hip complex.
The abdominals also play other roles. They contribute to the integrity of the vertebral column. In fact, weak abdominal muscles are often responsible for low back pain caused by poor intervertebral disc alignment. The abdominals also can aid in exhalation. When they contract, they squeeze on the underlying organs that push up against the diaphragm to increase intrathoracic pressure and help push air out of the lungs. And most people can appreciate the contribution of the abdominals in evacuating the bowels from the last time they had a lower gastrointestinal flu.
Those who choose to look further into abdominal exercises and core fitness will find dozens of exercises designed to activate very specific areas of the core such as the upper, middle, or lower abs. Such specificity will ensure that every aspect of each muscle is activated. It is easy to get both overwhelmed with the exercise options and carried away with implementing more activities at the expense of technical and tactical training for the game. Athletes are encouraged to perform their core training at a time apart from formal team training, reserving a few core exercises for the warm-up.